The invention concerns a method for the dynamic control of the converter process, especially during the blowing of oxygen in steelmaking by means of exhaust as analyses.
Modern steelmaking processes require exact knowledge of the current composition and temperature of the liquid metal being processed. Especially during steelmaking in a converter, high match rates for the final carbon content and the bath temperature are necessary. Quantitatively precise blowing of oxygen corresponding to the final target carbon content and the metal temperature are decisive factors affecting the economy of the process and the quality of the steel produced.
In order to realize relatively precise process monitoring, various procedures and processes are known, whose application is based in most cases on mass flow balances and the measurement of exhaust gas composition. All models work with a precision that depends on the exactness of the input data, especially with respect to the weight data of the raw materials and the chemical properties of the metal that is being used.
DE 28 39 316 A1 discloses a method for controlling a steelmaking process, in which the decarburization of the molten steel takes place under atmospheric pressure and with the formation of an exhaust gas containing CO, CO2, and N2. This method has the following steps: the formation of an intimate gas mixture from the exhaust gas and a measured quantity of a reference gas that is inert relative to the exhaust gas, mass spectrometric monitoring of a sample of the intimate mixture for the ionization currents for selected peaks related to the CO, CO2, N2, and reference gas in the sample, determination of the rate or amount of decarburization of the molten steel at the time of the monitoring from the measured value of the quantity of reference gas in the mixture and the measured values of the ionization currents for the selected peaks, and control of the steelmaking process according to the determined value of the rate or amount of decarburization of the molten steel.
WO 2008/049673 A1 describes a method for automatically controlling the CO output of the steelmaking process, in which oxygen (O2) is introduced into the melt to remove the carbon (C) present in the steel, the actual value of the carbon stream escaping from the melt is determined, a desired value for the escaping carbon stream is calculated on the basis of the amount of oxygen supplied and the carbon content of the melt, taking in account any other reactions, the desired value and the actual value are compared, and if the actual value is lagging behind the desired value, measures are undertaken to prevent the abrupt rising of gas bubbles. The following are specified as suitable measures:                control of the oxygen supply to the melt and possible reduction of the oxygen supply,        supplying carbon to the melt.        
WO 2009/030192 A1 discloses a method for indirect determination of the exhaust gas rate in metallurgical processes. This method is characterized in that first a reference gas, such as helium, is added to the exhaust gas, specifically, at a point in time which precedes the taking of a sample to a sufficient extent, with respect to flow, that the reference gas and the exhaust gas have become thoroughly mixed, i.e., a virtually homogeneous distribution has been achieved, and that then a quantitative helium and nitrogen analysis is undertaken, measured with a mass spectrometer, taking into account the amount of helium added, with the following individual determinations: O2, CO, CO2, N2, Ar, He, H2.
As explained in a paper in “Stahl and Eisen 113 (1993), No, 6, p. 56,” more than 20 years ago engineers began using the information about the decarburization process in the LD converter that is contained in the exhaust gas. For 9 of 17 basic oxygen steelmaking plants named in a review through 1980, an exhaust gas measurement was specified for process control of oxygen converter processes and an increased setpoint setting accuracy of 85% to 95% was given for a range of carbon content of ±0.020%. This corresponds to a standard deviation of the error of about 0.014% to 0.010%. In the meantime, exhaust gas measurement has become, as will be explained further—partly with mass spectrometer, partly with individual instruments—a permanent part of most process control systems for LD converters and the oxygen steelmaking processes derived from them. In supplementation of the so-called “static model” for precalculating the process, the exhaust gas measurement, together with the sublance, makes possible the continuous observation and control of the process in a so-called “dynamic model.” The decreasing decarburization rate before the end of treatment indicates when a sublance measurement is to be performed in order to reliably hit not only the target carbon content but also the target temperature with this position determination. The calculation of the slagging of iron, manganese, phosphorus and sulfur on the basis of the oxygen balance can determine the bath composition so exactly that most melts can be directly tapped and alloyed without taking a control sample and awaiting its analysis.